1. Field of the Invention
The present invention relates to a subtractive color method applied to image data that has been scanned by a scanner, and an image forming apparatus etc. that perform subtractive color processing.
2. Description of Related Art
In recent years, digitization of information has been progressing, and it has become common to digitize information recorded on a paper medium for storing. When performing digitization, a document is scanned using a scanner, and image data obtained by scanning is converted into a predetermined storing format and stored as digitized data. Presently, PDF (Portable Document Format) is widely used as the storing format. When the image data is converted into a file in PDF format and stored, an amount of data thereof can be greatly reduced. However, when a file in PDF format (PDF file) is generated, usually, compression using the JPEG (Joint Photographic Experts Group) method is performed on the image data. As a result, a slight degradation in image quality is inevitable. Accordingly, to maintain a compression ratio unchanged while the image quality is kept as high as possible, there is proposed a method for performing compression that is suited to each area included in an image (target image) represented by the image data to be stored. According to this method, a PDF file is generated, for example, in a way as described below.
First, an area represented by an object such as text, photograph, table, graph, or graphics included in the target image is extracted. Then, it is determined whether the extracted area is a text area or a non-text area. Binarization is performed on the text area, and one representative color is decided for the text. Further, compression processing by taking the legibility of the text into account is performed. Compression processing at a high compression rate is performed on a photograph area. Image processing such as subtractive color processing is performed on an area that is neither the text area nor the photograph area (hereinafter, referred to as “graphics/table/graph area”), namely, a table area, a graph area, or a graphics area, and compression is performed thereon with a high resolution maintained. Thereafter, a PDF file is generated by using the compressed data of individual areas.
With this arrangement, it is possible to output a PDF file having a higher image quality while having a similar amount of data as in the conventional method.
As shown in FIG. 18, an image of graph has the characteristics of including many of portions that are supposed to be expressed by a single color such as a bar portion AB in a bar graph and a background portion AH serving as a background to the bar. A table and graphics also have similar characteristics. Usually, when a document is scanned, a plurality of colors are included in such an area which is supposed to be expressed by a single color because of uneven density in color. Accordingly, for such an object including many portions that are supposed to be expressed in a single color, such as graph, table, and graphics, the image quality and the compression ratio are expected to be improved particularly by performing the subtractive color processing thereon. As an art relating to the subtractive color processing, such technologies as described in JP-2002-300412-A and JP-H06-103371-A are proposed.
According to a method described in JP-2002-300412-A, an inputted image is divided into small areas by a predetermined method, and colors are reduced by reducing halftone levels in each of the small areas.
According to a method described in JP-H06-103371-A, a plurality of representative colors are selected from a color image to be processed, and colors that are not selected as the representative colors are substituted by representative colors most closest thereto, respectively. The color image is divided based on the representative colors to obtain a plurality of partial image areas. If it is determined that a change in image quality is not visually recognized when one of the partial image areas and another adjacent thereto are integrated, the one of the partial image areas is integrated into the another of the partial image areas adjacent thereto. The integration process is repeated for each of the partial image areas to thereby reduce colors in the entire color image.
According to the conventional method described in JP-2002-300412-A explained above, since colors are reduced by reducing the number of halftone levels, areas having similar colors may be converted into areas having an identical color. As a result, in the case where there are a plurality of bars having similar colors but different meanings, for example, as in the case of FIG. 18, the plurality of bars could be converted to the bars in an identical color. If such conversion is performed, the significance of the graph is changed. Therefore, this method is not suitable for reducing colors in the graphics/table/graph area.
According to the conventional method described in JP-H06-103371-A explained above, one of partial image areas and another of partial image areas adjacent thereto are compared, and it is determined whether or not a change in image quality can be visually recognized if the two areas are integrated. This way of determination is performed on each of the partial image areas that are adjacent to the one of the partial image areas. Then, if it is determined that no change in image quality is visually recognized, the integration is performed. Since this integration process is performed further for all of the partial image areas one by one, an amount of calculation increases, and it may take a long time to process.